1. Field of the Invention
The present invention relates to a silicon-containing alumina support, a process for preparing it and a catalyst containing it, especially a catalyst for hydrotreating hydrocarbons.
2. Description of the Related Art
Alumina is a commonly used support for catalysts. Its surface properties, such as the specific surface area, the amount and strength of the surface acid, and porosity properties, such as the average pore size and the pore size distribution, have decisive influence on the performance of the catalysts containing it. In order to improve the properties of alumina supports, silicon, phosphorus, boron, titanium and zirconium are used to modify it. Of the modifying agents, silicon is the most commonly used. It has been found that silicon modifies the surface properties of alumina support. However, silicon introduced into alumina support in accordance with the prior art adversely affects the microstructure of the alumina support, e.g. renders the pore size distribution of alumina support diffuse, adversely affecting the surface properties.
Alumina support is commonly used in catalysts for hydrotreating hydrocarbons. In the catalysts for hydrotreating hydrocarbons, the active components are the oxides of a metal from Group VIB and a metal from Group VIII of the Periodic Table of Elements. To improve the performance of the catalysts, one approach is to modify the alumina support. It has been shown that silica introduced into the alumina support has significant effect on the catalyst containing the alumina support in terms of acidity, the dispersion of the metals and pore size distribution. Specifically, in accordance with the theory of minimum energy, the silica introduced into the alumina support first interacts with the strong active centers on the alumina surface and reduces the strength of the latter. As a result, the interaction between alumina support and the active metals become milder, thus resulting in the metals being dispersed better and displaying better activity. Therefore, one of the strategies to improve the catalysts for hydrotreating hydrocarbons is to modify the alumina support with silicon.
One of the major sources of silicon for introducing into catalyst for hydrotreating hydrocarbons is silicates or silicon sol. To introduce the silicon, an aluminum-containing compound is co-precipitated with sodium silicate (or water glass) to give a silicon-containing alumina; or alternatively, aluminum hydroxide or alumina is mixed with a silicon sol to give a silicon-containing alumina hydroxide. Typically, the content of silica in the catalyst is 1 to 10% by weight.
U.S. Pat. No. 4,758,544 discloses a process for preparing by co-precipitation a support, i.e. alumina-aluminum phosphate-silica, and loading the support with W—Ni—F to give a catalyst. Since the elements silicon and phosphorus are introduced into the support by co-precipitating, they are inevitably dispersed in the bulk of alumina support in large amount, thus reducing the availability and effect of them. In addition, the presence of both silicon and phosphorus in the alumina support affects the microstructure of the alumina support, leading to a diffuse pore size distribution, a small pore volume and specific surface area. Thus, the dispersion and effect of the additives (P and Si) suffer.
Chinese Patent No. 1048651C discloses a process for preparing an alumina support containing silicon and phosphorus by precipitation from aluminum sulfate and sodium meta-aluminate. Since the silicon source is sodium silicate, there exist defects similar to the above with this process.
U.S. Pat. No. 4,134,856 discloses a process for preparing alumina support containing silica by co-precipitation. There also exist defects similar to the above with this process. In particular, the alumina support prepared by the process has a diffuse pore size distribution, and a large specific surface area and pore volume, and is amorphous.
Chinese Patent No. 1030395C discloses a process for preparing a support or catalyst wherein silicon sol is introduced into aluminum hydroxide during the procedures for moulding alumina support or catalyst. However, silicon sol acts to expand the pores in the alumina support or catalyst in the subsequent drying or calcining procedures. Therefore, the obtained alumina support or catalyst has a diffuse pore size distribution, unfavourably affecting the performance of the catalyst.
Therefore, the problem common to the prior art processes for introducing silicon of the indicated source into alumina support is that the content of silicon in the bulk alumina support is relatively high, while that on the surface is relatively low, adversely affecting the effect of silicon modifying the surface properties of the support.